Laser anemometers are used in particular in aircraft for measuring the velocity of the aircraft with respect to the ambient air which contains natural particles in suspension. One of the advantages of laser anemometers over the anemometric probes which measure pressure is that they make it possible to measure the velocity with respect to the ambient air not directly on the surface of the aircraft, but at a certain distance from this surface, where the air is disturbed less by the aircraft itself or even not disturbed at all.
Distinction is made between two main types of laser anemometers: transverse-measurement anemometers and longitudinal-measurement anemometers.
Transverse-measurement anemometers emit two beams of coherent light, which are slightly inclined relative to one another and which interfere in a measurement zone (for example at a distance of about one metre from the emission optics). Plane-parallel interference fringes are produced in this zone, and a particle which crosses these fringes undergoes illumination with a sinusoidal overall intensity; it therefore reflects a quantity of light which varies sinusoidally. The frequency of this variation depends on the velocity component of the particle in the direction perpendicular to the plane of the interference fringes. This direction is situated in the plane defined by the two inclined beams and perpendicular to the bisector of these two beams. These anemometers therefore measure a velocity component which is transverse with respect to the overall axis of the beams.
Longitudinal-measurement anemometers operate on a different principle. They emit a single laser beam and measure the relative-velocity component of the particles in the direction of the optical axis of this beam. The beam is focused by emission optics at a large distance (for example 50 metres) into a measurement volume where the illuminated particles will return a small fraction of energy towards the source in coherence with the emission beam but with a Doppler-type shift in respect of the optical frequency, which is due to the velocity component of the particles in the return axis of the reflected beam. The detection consists in making the back-scattered coherent light interfere with a fraction of the emitted beam, and in producing intensity beats at an electronically detectable frequency. The electronic signal is then processed in order to extract a frequency spectrum from it and to deduce from this spectrum a statistically dominant component which represents the average velocity of the particles with respect to the anemometer in the direction of the optical axis. These longitudinal-measurement anemometers make it possible to measure the velocity of an aircraft with respect to an ambient-air zone which is far away from the aircraft, and which is therefore disturbed little by it, whereas transverse-measurement anemometers observe the air at a distance of no more than about one metre, and therefore in practice in a turbulent atmosphere.
When it is desired to measure a complete velocity vector, use is made of three successive or simultaneous anemometric measurements whose optical axes are oriented along three known directions (for example three orthogonal directions), and the velocity vector is determined according to its three components. More measurements (for example four) may be used in order to obtain redundancies which reduce the measurement error of the velocity vector. This is typically the case in an aircraft where the velocity with respect to the air both in the horizontal plane and along a vertical axis is useful.
One drawback of laser anemometers compared with simple pressure-measurement probes is that they are bulky and, for this reason, difficult to install in aircraft. For example, it is not easy to find a place on the aircraft where, on the one hand, there is space to install the anemometer and, on the other hand, the three objectives do in fact point towards directions of interest, which are preferably orthogonal.
It is consequently an object of the invention to design an anemometer which can be installed as easily as possible while taking into account the constraints dictated by the environment.
For this purpose, it has already been proposed in Patent FR-A-2 659 452 to place the laser source, and its focusing optics, at a distance from the interferometric-detection and electronic-processing means, and even at a distance from the optical-pumping source of the laser when the laser is optically pumped. However, in the general case when a measurement of the velocity vector is taken along three axes, this would make it necessary to use optical switches which are difficult to implement.
It has also been proposed in Patent FR-A-2 761 162 to use optical fibres and optical couplers in various parts of the system, but the architecture which results from this can operate only for very low light powers, of the order of about 1 watt. For certain applications, such as the observation of velocities at a large distance and at high altitude, these powers are much too weak and powers at least ten times higher are desired.
The invention consequently provides a novel general architecture of a laser anemometer, allowing greater installation flexibility, ease of production, but nevertheless a sufficient emission power, while furthermore taking into account other technical constraints inherent in optical and electronic systems (minimizing the optical noise and the electronic noise, for example) and while of course taking into account cost constraints, in particular for mass production.